hmmm it could be a bug, but proving that and getting it fixed will be an uphill battle - you need to positively eliminate as much as possible first. Get a new/known-good cat5/6 cable and connect one of these machines directly to the core (no switch) - depending on the NICs a normal cable should be fine but if you have a cross-over then use that. This will eliminate your network and switches.
Do you have any spare NIC cards (different brand, preferably one mentioned on the wiki as tested) - try substituting you Internal NIC for this card. Also, please confirm Roy's question on your network setup. You have 2 NICs, one internal one external, and the XP machines are connected to the internal NIC. The internal network has no other DHCP server on it (are there any other devices at all?). The internal and external networks/NICs are not connected to the same switch or any other way to mix the 2 segments?
If no other NICs, then check out your current NIC with the wiki to see if anybody has confirmed compatibility (or at least on a Linux site and look for the latest driver)